1. Field of the Invention
The present invention relates to optical scanners in general and more particularly to optical scanners capable of reading high density bar codes over a wide range of distances from the scanner.
2. Prior Art
Optical bar code scanners for use in retail checkout operations at supermarkets and other retail establishments are gaining wider and wider acceptance. The typical bar code scanner is usually mounted within a counter and has a window at the top thereof through which a scanning pattern is projected. The scanning pattern is formed by a laser and associated optical components which produce plural scan lines that intersect in various patterns. If an article imprinted with a bar code, such as the Uniform Product Code (UPC) is placed into the field of the scan pattern and the bar code is scanned by the scan pattern, light is reflected from the bar code back through the scanner window.
A photodetector converts the reflected light into an electrical signal which is applied to dedicated hardware or to a microprocessor which finds and decodes the symbol into a stream of electrical signals. When the signal has been found and decoded, it is passed onto a point of sale terminal or store controller where it is used to access records that provide a brief description of the item as well as pricing information. The pricing information and item description are used at the point of sale terminal to prepare customer receipt and to calculate transaction totals.
In order to ensure the continued acceptance of scanners as a vital tool in the retail business, there has been a constant effort on the part of manufacturers to improve the performance of these scanners. One area which is being improved is the scanner's "depth of field." The "depth of field" of a scanner is the range of distances over which the scanner can successfully read the smallest bar code label allowed by the standards authority for the particular bar code being read. The range of distances usually extends from the scanner window to one or more focal points in space. The focal points are points in space where the scanning beam is focused by the scanner's optical system.
Stated another way, the "depth of field" of a bar code scanner is an indication of the distance, relative to the scanner window, whereat a bar code label can be placed and still be read by the scanner. By designing the scanner with a large depth of field an operator, at a checkout stand, is free to position an article at different distances from the scanner window and still have the bar code read by the scanner. This freedom tends to improve the productivity of the operator and also ensures the continued acceptance of the scanner.
The prior art has provided scanners with single and multiple focal points. U.S. Pat. No. 4,713,532 to Carl H. Knowles discloses a scanner with a single focal point. The scanner has a symmetrical rotating deflector which deflects and focuses a laser beam at a single point above the window of the scanner. The deflector is formed from a plurality of angled single faceted mirrors. Due to the single focal point, this type of scanner has a relatively small "depth of field."
Scanners with multiple "focal points" and relatively large "depths of field" are set forth in U.S. Pat. Nos. 4,748,316, 4,591,242 and 4,560,862. In U.S. Pat. No. 4,748,316 a rotating holographic disc is used to create the different focal points. In U.S. Pat. No. 4,591,242 and 4,560,862 complicated configurations which include optical lenses, multiple lasers, modulators, etc. are used to generate the multiple focal points. Even though these scanners work well for their intended purpose, the drawback is that they tend to be expensive, unreliable, and difficult to repair when breakdown occurs.